Effective control of Chagas' disease or South American trypanosomiasis, caused by the monocellular flagellate Trypanosoma cruzi, requires a better understanding of the host defense mechanisms against this parasite. The major emphasis of this project is placed on the immune mechanisms which produce destruction of virulent forms of T. cruzi. The prposed study will be directed toward: 1) Establishing the relative contribution of cell-mediated immunity to host defense mechanisms effective against T. cruzi infection. Alternaive approaches are proposed to resolve this question based on the use of cell transfers of sensitized T and B lymphocytes as well as of animals in which only one of these cell claseses will be immunologically competent. 2. Determining the susceptibility of trypomastigote forms of T. cruzi (found in infected hosts) to destructive, cell-mediated cytotoxic mechanisms. 3. Defining the effects of selective cell-mediated or humoral immunosuppression on experimental Chagas' disease.